Scanning Electron Microscopy

Overview

Scanning Electron Microscopy (SEM) utilizes a high energy beam of focused electrons to generate images or micrographs of the topographic detail and/or material variation in small specimens. SEMs are capable of few-nanometer resolution with the appropriate sample and operating conditions.

Equipment

• JEOL 7500F HRSEM
• TFS Quanta 600 FEG ESEM

Techniques

• EDS
• EBSD
• Low Voltage SEM
• Low Vacuum SEM

Key Principles

Conductivity

Most SEMs perform best when imaging conductive samples. Conductive samples allow electrons from the beam to travel freely to ground if they are not emitted as signal. Non-conductive samples may be sputter coated with a few nanometers of conductive material to improve images.

Exceptions include: Low Vacuum SEM and Low Voltage SEM which compensate for the lack of conductivity with either charge balance and signal amplification via ionized water vapor or reduced incident electron energy.

Scanning

SEM images are generated one pixel at a time as the electron beam rasters, or scans, across a specified region. Slower scans, with longer dwell times, result in more signal, but may introduce artifacts from charge buildup. Short scans tend to appear very noisy due to a low signal-to-noise ratio (SNR.)

Beam-Sample Interactions

Incident electrons are either scattered or absorbed by the sample.

Absorbed electrons travel through conductive specimens through the stub, sample holder, and stage to ground. However, these electrons may appear as charge buildup in less conductive specimens, or specimens not well grounded to the stub or sample holder.

Scattered electrons produce signal.

• Secondary electrons encounter multiple inelastic scattering events before escaping through the specimen surface. These yield topographic contrast and come from the first few nanometers of the sample surface.
• Backscattered electrons elastically scatter once before escaping the surface. These yield material contrast and are generated from up to 100nm below the specimen surface.
• Characteristic X-Rays are emitted when a higher energy electron falls to the energy level of an electron displaced by the incident beam. These x-rays can identify specific elements and may be generated from up to 1um beneath the specimen surface. Characteristic x-rays are fundamental to EDS analysis.

Signal Collection

Emitted signals (secondary electrons, backscattered electrons, characteristic x-rays) are each collected by dedicated detectors, which may have a bias applied. SEM detectors collect all the signal in the chamber for a given point, and computer controls correlate the pixel of the image with the scan location of the beam. Images may appear to have shadows if the path between the point of interaction and the detector is blocked by another portion of the specimen topography.